Waveguide antenna with dielectric guiding structure at aperture



April 21,, 1979 ROPE ET AL 3,508,276

7 WAVEGUIDE ANTENNA WITH DIELECTRIC GUIDING STRUCTURE AT APERTURE Filed Oct. 17, 1966 I] INVENTORS. I /TO RADAR I] GUS PETER TR/COLE'S SYSTEM [1 EUGENE LOWELL Rap 7 QZ 6W0 United States Patent 3,508,276 WAVEGUIDE ANTENNA WITH DIELECTRIC GUIDING STRUCTURE AT APERTURE Eugene L. Rope and Gus P. Tricoles, San Diego, Calif.,

assignors to General Dynamics Corporation, a corporation of Delaware Filed Oct. 17, 1966, Ser. No. 587,284

Int. Cl. H01q 13/10 U.S. Cl. 343-771 11 Claims ABSTRACT OF THE DISCLOSURE A microwave antenna which readily provides monopulse reception is described. The antenna includes a dielectric slab sandwiched between two open-ended wave guides and extending outwardly from their open ends. When an incoming wave is incident on the slab, it causes two guided waves to be excited and propagated towards the open ends of the wave guides. Unguided waves also pass directly into the open ends and after refraction through the slabs. The waves interfere with each other and the open ends may be placed at a position of maximum power, such that two separate signals are induced, one in each guide. These signals may be combined to provide sum and difference patterns for monopulse operation.

This invention relates generally to antennas, and more particularly, to antennas having at least one dielectric element and which are adapted to operate in the microwave radio frequency range.

One version of dielectric antenna which has long been used with some degree of success employs a single slab of a dielectric material which is disposed in the open end of a wave guide. Such an antenna makes use of a wave guided by the dielectric element and a wave which passes into the dielectric slab, and at the mouth of the wave guide combines with the guided wave to produce a signal which is then coupled by the wave guide to a radar system for appropriate analysis.

Although this type of antenna has great usefulness in that it is compact and can be mounted in small areas, it is subject to disadvantages, one such disadvantage being that its ability to pick up weak signals is somewhat limited.

In view of the foregoing, an object of this invention is to provide a dielectric antenna, small in size and inexpensive in construction, which eliminates the above-indicated problem.

A further object of this invention is to provide a microwave antenna which includes a minimum number of easily assembled components and to which received signals may be easily and efliciently coupled.

A further object of this invention is to provide an antenna which is especially suitable for use with monopulse type radar systems.

A still further object of the present invention is to provide an antenna, the mechanical design of which permits it to be mounted so that it will closely conform to the surface upon which it is afiixed.

Briefly described, an antenna embodying the present invention makes use of the interaction between waves having both guided and unguided modes of propagation to develop a signal as did the above-described prior art arrangement, but in addition, it makes use of a refracted wave which passes completely through and out of a dielectric element and thereafter combines with the waves which propagate in the first two modes.

One exemplary embodiment in accordance with. the invention employs a dielectric element which is sandwiched between two open ended wave guide members. The

3,508,276 Patented Apr. 21, 1970 dielectric member extends from its sandwiched position where it engages the wave guides outwardly to a free end which is spaced from the wave guides. Each of the wave guides supports various modes of propagation. One of the wave guides develops a signal which isthe result of the interaction between waves having three modes of propagation: a wave guided by the slab; an unguided wave which combines with this guided wave; and an unguided wave which, after refracting through the dielectric element, passes directly into the open end of the wave guide.

Another embodiment in accordance with the invention includes two spaced dielectric members and a wave guide to which these dielectric members are aflixed. Preferably, the wave guide is of rectangular configuration and includes a surface which is perpendicular to both of the planes of the dielectric members and in which are cut a series of slotted openings. The antenna operates in a manner similar to that in the first embodiment and therefore permits waves having the above-described three modes of propagation to be coupled to the interior of the wave guide.

An important feature of antennas made in accordance with the invention is that they have directional narrow beam patterns.

Another important feature of the invention is that antennas made in accordance therewith are especially suitably for use in terrain following radar systems and because of their compact design are readily mounted in very small areas, such as the tip of a helicopter blade.

The invention itsel f, both as to its organization and method of operation, as well as additional objects and advantages thereof, will become more readily apparent from a reading of the following description, taken in combination with the accompanying drawings, in which:

FIG. 1 is a diagrammatic illustration, partially broken away, of an exemplary microwave antenna in accordance with the present invention;

FIG. 2 is a side view of the antenna shown in FIG. 1, which shows by means of arrows the various modes of propagatlon which it receives;

FIG. 3 is an elevational view of another antenna embodiment in accordance with the invention; and

FIG. 4 is a front view of the antenna shown in FIG.

Referring to FIGS. 1 and 2, there is shown an antenna 10, comprising two open-ended wave guide members 12 and 13, each of rectangular cross section, and a plate element 14 formed of a refracting dielectric material such as polyethylene. The element 14 is rectangular in construction, having two parallel planar surfaces 14a and 14b. One end of the dielectric element 14 is sandwiched between the members 12 and 13 and may be fixed to them by means of an adhesive such as an epoxy resin. From this fixed end the element 14 extends outwardly to a free end portion 14c which is spaced from and substantially parallel to the planes formed by the open ends of the wave guides 12 and 13.

In operation an unguided wave, when incident upon the free end and the planar surfaces 14a and 14b, causes a guided Wave 15 to be excited and guided. More particularly, the wave 15 is guided along a planar surface of the member 14. For the sake of convenience of illustration, all of the waves have been shown as rays by means of arrows.

At different positions along the surfaces 14a and 14b of the member 14, an incident unguided wave is composed of two positions 16 and 17. The portion 17 does not pass through the member 14; the portion 16 is refracted through the member 14. The unguided wave 16 and 17 will interfere with the guided wave 15 and produce regions of maximum and minimum power. As illustrated, the portion of the wave 16, refracted through the element 14, will pass into the open end of the wave guide 12. Accordingly at the open end of the wave guide 12, there will be waves having two modes of propagation which will combine to produce a signal. The wave guide, of course couples the signal to an appropriate radar systern for analysis. Summarizing, the two waves having two modes which are coupled to the wave guide 12 are: the guided wave 15 and the refracted portion of the incident wave 16, which passes through the element 14 into the opened end of the wave guide 12.

The wave guide 13 will, of course, receive an unguided wave illustrated by the numeral 17 and the guided wave 15 which propagates along the surface 14a. For an informative discussion of the interaction of guided and unguided waves in a dielectric slab, see Tricoles and Rope, J. Opt. Soc. Am., vol. 55, p. 328 (March 1965).

By varying the position of the member 14 (viz. changing the distance of its free end 140 of the wave guides 12 and 13), the antenna may be tuned to operate at different frequencies. Stated another way, the wave guides are placed at a position of maximum power when the antenna is tuned to an optimum condition.

A particular advantage of the antenna 10 is that it may readily be incorporated within a monopulse radar system inasmuch as two separate signals are induced, one within each wave guide 12 and 13, when an incoming signal is intercepted by the antenna. In monopulse operation, each of these wave guides may, for example, then be connected to conventional phase shifting mechanisms and then onto a hybrid junction where sum and difference patterns may be developed.

FIGS. 3 and 4 of the drawings depict still another representative antenna 30, especially suitable for use at microwave frequencies, which includes a line source radiator, namely, an elongated rectangular slotted wave guide member 31. In order to produce a desirable radiation pattern, a plurality of slots 32 are cut in a side planar surface 33 of the member 31 and are preferably spaced one-half a wave length apart at the operating microwave frequency, the spacing being measured from center to center between alternate slots. Because of this spacing of the slots 32, the antenna 30 operates in the TE mode and radiation delivered from each slot 32 is in phase. In practice, in order to insure a uniform radiation pattern, the slots 32 may take other geometric shapes; for example, they may be dumbbell-shaped (viz. rectangular slots with small circular holes formed at the remote ends).

The wave guide 31 is connected to a radar system at one end and placed at its other end is either a dissipative material or an adjustable waveguide shorting plate device. Briefly a dissipative termination is, of course, one which is able to absorb power and dissipate it in the form of heat; whereas a shorting plate device may be provided by a holder mechanism carrying a metallic rod which is adapted to vary the position of the rod within the interior of the wave guide. By proper positioning of the rod, a reflected wave will be excited in a proper phase relation so as to yield optimum antenna radiation pattern. For the purpose of illustration, a tuning micrometer mechanism 35 has been shown to provide this function of the holder mechanism. It will be understood, however, that other conventional means for adjusting the impedance of the wave guide 31 may also be suitable in the practice of this embodiment.

The antenna 30 also includes two spaced members 36, each member 36 being formed of a non-ferrous refracting dielectric material, such as Plexiglas (viz. the trademark for the cast acrilic resin methyl methacrylate) or fiberglass. Moreover, each of the members 36 is rectangular in configuration and includes two parallel surfaces 40, through which an incident wave is refracted. In addition, each member is bonded adjacent to one edge of the radiated surface 33 of the wave guide 31, say by means of an epoxy resin and extends outwardly from its bonded position to a free edge 42, which preferably is parallely disposed in relation to the radiating surface 33 of the wave guide 31.

At this point, it will be noted that although the illustrated antenna 30 uses two members 36, the antenna 30 may be successfully practiced in accordance with the invention with the use of only one such member 36.

In operation, two waves having three possible modes of propagation are propagated by the antenna 30. The first wave is a plane unguided wave which is incident upon and interacts with the guided wave at a position adjacent to the slot 32 and a portion of which refracts through the parallel surfaces 40 of one of the dielectric members 36, and which thereafter encounters the slotted holes 32. Of course, in a specific case, if the received unguided wave is substantially perpendicular in relation to the surface 33, it will encounter the slots 32 without encountering surfaces 40. The second wave is the guided wave caused by the first wave being incident upon and illuminating the free edge 42 and the surfaces 40 of the member 36. These waves are propagated along the interior surfaces of both of the dielectric members 36 up to the slotted radiating surface 33 of the wave guide 31.

With the above-described embodiments, the three modes of propagation give rise to interference and produce a pattern of discernible maximum and minimum power levels at discrete positions along the surfaces of the dielectric members. Moreover, the maximum and minimum power level points will vary with the angle at which a microwave beam is incident upon these surfaces. The position will also vary depending upon the wavelength, and the incident Wave and the dimension and the dielectric constant of the material forming the members 36. A number of substantially equally spaced maximum and minimum power conditions will be observed at any selected position as the angle of incidence of a microwave beam and the surface of a dielectric member is varied. Thus, by locating the slot at the maximum power points, a desired directional response is obtained.

While two embodiments of the invention have been shown and described, it is to be understood that changes may be made within the scope of the invention. For example, the invention is not limited to antennas employing wave guide type receptors, but also may employ other probes such as dipoles, Yagi arrays, and other line source type arrays such as a pillbox. Further, the invention is not limited to rectangular dielectric slabs, but may also employ hollow dielectric cone shaped members which develop interfering guided and unguided wave patterns.

What is claimed is! 1. An antenna comprising:

(a) an elongated dielectric member comprised of a material which, in response to an incident wave, refracts through said dielectric member, at least a portion of the wave and causes a guided wave to be generated and guided along a surface of the member, said member having a forward end and sides extending rearwardly from said forward end, said guided and unguided waves interfering to produce discrete positions of maximum and minimum power along said dielectric member surface from said forward end rearwardly thereof, and

(b) coupling means disposed at one of said discrete positions to the rear of said forward end of said dielectric member, said coupling means being spaced from said forward end of said dielectric member and located entirely to one side of said surface for receiving and conducting electromagnetic waves which are the combined effect of said interference between said unguided and guided waves and said unguided wave portion refracted through said dielectric member.

2. The invention as set forth in claim 1, wherein said dielectric member is an elongated slab, and wherein said coupling means is a line source radiator.

3. The invention as set forth in claim 2 wherein said line source radiator is disposed in parallel relation to an edge of said dielectric member.

4. The invention as set forth in claim 3, wherein said line source radiator is a slotted wave guide having a slotted planar surface which is disposed in parallel relation to said edge of said dielectric member.

5. The invention as set forth in claim 4, including an additional dielectric member, fixedly secured adjacent to the slotted surface of said wave guide and spaced in parallel relation from the first said dielectric member.

6. The invention as set forth in claim 5 wherein both said dielectric members are rectangular in configuration.

7. The invention as set forth in claim 1 wherein said coupling means is com-prised of a first and second wave guide member.

8. The invention as set forth in claim 7 wherein said wave guides are open-ended and wherein said dielectric member is sandwiched between said first and second waveguides.

9. The invention as set forth in claim 7 wherein said dielectric member is rectangular in configuration.

10. The invention as set forth in claim 9 wherein said dielectric member includes a free end disposed in parallel relation to the planes formed by the open ends of said wave guides.

11. The invention as set forth in claim 1 wherein said coupling means is disposed at a distance from said forward end of said member where said interfering waves produce a signal which is at a relative maximum power condition.

References Cited UNITED STATES PATENTS 2,822,542 2/ 1958 Butterfield 343-785 3,392,396 7/1968 Ehrenspeck 343-785 FOREIGN PATENTS 937,781 1/ 1956 Germany.

ELI LIEBERMAN, Primary Examiner U.S. Cl. X.R. 343778, 785 

